Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact imaging device, and more particularly to an imaging lens which is built in an imaging device mounted in an increasingly compact and low-profile mobile phone, smartphone, or PDA (Personal Digital Assistant), or a game console, or an information terminal such as a PC and a robot, or a home appliance or a car with a camera function.
Description of the Related Art
In recent years, it becomes common that a camera function is mounted in many information terminals. Furthermore, it becomes requisites to mount the camera in mobile terminals, such as the mobile phone, smartphone, or PDA (Personal Digital Assistant) as an additional value to the products. Not only the mobile terminals, there has been increased demand of products to which a camera function is added, such as a wearable device, a game console, PC, a home appliance, and a drone, and development of such products may be rapidly proceeded. Recently, a display device built in the above information terminals becomes large and achieves high-definition, therefore high pixelation is required for an imaging lens. The imaging lens to be mounted is required to have further high-performance.
Demand of compactness in the imaging device is still strong. Therefore, regarding the image sensor, in order to realize compactness while maintaining high pixel, micronizing of pixel size has been rapidly proceeded. If the pixel size of the image sensor becomes small, received luminous quantity per one pixel is decreased and deterioration in image quality by noise becomes serious problem. In order to solve the problem, a bright optical system is required for the imaging lens, and demand of a lens having a large diameter of F1.9 or less is increased.
However, realization of the imaging lens satisfying low-profileness and low f-value at the same time difficult, especially aberration correction in a peripheral area is difficult. Therefore, there is problem to secure proper optical performance throughout the image.
There is known a conventional imaging lens composed of eight constituent lenses, such as that disclosed in Patent Literature 1.
Patent Document 1 (JP-A-2001-13405) discloses an imaging lens having eight constituent lenses, which is composed of, in order from an object side, a first lens group having negative refractive power, a second lens group having positive refractive power and a third lens group having positive refractive power. The first lens group comprises, in order, a positive lens and a meniscus negative lens having a convex surface on an object side, the second lens group comprises, in order, a positive lens, a negative lens and a positive lens, and the third lens group comprises, in order, a double-sided concave lens having a strong curvature on the object side, a meniscus positive lens having a convex surface on an image side, and a positive lens. An aperture is provided between the second lens group and the third lens group.
The imaging lens disclosed in the above Patent Literature 1 is related to an exchange lens for a 35 mm single lens reflex camera, which has only eight constituent lenses without using aspheric surfaces, and large diameter and high-performance and is compact, and its purpose is to largely reduce cost in production. This lens achieves brightness of F2.1. The total length becomes 50 mm or more because it is used for a single lens reflex camera, and it is very difficult to apply to the mobile terminals or information devices. If there are realized low-profileness and further low F-value, by means of the lens constitution disclosed in Patent Literature 1, all of surfaces are spherical and it is difficult in aberration correction in a peripheral area and it can not be obtained high optical performance required in recent years.